8/3/2019 Guidelines for Earthing
1/2
Guidelines for Earthing
1. Minimum fault level at 11 kV side shall be assumed 150 MVA
for all earthing designs.
2. Number of earth electrodes required shall be calculated for
fault current at 11 kVside.
3. Duration of fault current shall be taken as 3 sec for
earthing design at HTinstallations.
4. Maximum earth resistance at HT premises shall be limited to 1
ohm.
5 Earth mat shall be provided at EHT premises for limiting step
voltage and touchvoltage to tolerable values. Conductor size and
mesh size may be designed for faultduration of 1.0 second and 0.5
second respectively.
6. All earth electrodes shall be interconnected using the
conductors of largest size in the
earthing system.
7. All non-current carrying metal parts of equipments shall be
double earthed usingconductors of adequate size.
8. Transformer and generator neutral shall be double earthed.
One independent earthelectrode shall be provided for neutral
earthing. Links shall also be provided in neutralearthing
conductor. Generator neutral shall be earthed at control panel.
9. Lightning arrestor shall be connected to a separate earth
electrode using No. 6 SWGCu. The L.A. earth electrode may be
interconnected with other earth electrodes.
10. If fault current exceeds 6 kA plate earth electrodes shall
be used.
11. If soil resistivity is < 60 ohm-m, only copper shall be
used as earthing conductor.
12. Distance between plate earth electrodes shall be 8 m
minimum.
13. Distance between pipe earth electrodes shall be 5 m
minimum.
14. Minimum distance between earth electrode and adjacent civil
structure shall be 1.5m.
15. In location of SBs, DBs etc. sub earth buses of 25 x 3 mm2
copper may be provided.
16. All joints in earthing conductors shall be properly
tinned.
17. Following guidelines shall be adhered to when GI earthing is
adopted.
(a) GI strips used for earthing shall be minimum 6 mm thick and
hot dip galvanised.(b) If round GI conductors are used it shall
have double the calculated area of cross-section.(c) The minimum
cross-sectional area of the GI should be 50 sq.mm.
8/3/2019 Guidelines for Earthing
2/2
(d) Where a Copper conductor is to be joined to GI, the joints
should be tinned toprevent electrolytic action.(e) If atmosphere is
corrosive, GI conductors shall not be used for earthing.(f) Joints
shall be invariably done by means of welding except in such
locations whereintermittent disconnection of joints is necessitated
for testing purpose, or for the earthingconnection to the body of
equipments etc.
(g) Barium Chromate or Zinc dichromate treatment should be done
at the welds. Jointsshould be provided with coating alternative
layers of red oxide and aluminium. Jointsare to be covered with hot
bitumen. Also jute band covering of adequate thickness is tobe made
on the portion of earthing susceptible to higher corrosion and
environmentalinfluence.(h) The area of strip electrode thus covered
with bitumen and jute should not bereckoned for the calculation of
area of dissipation for the fault current.(i) In places where the
resistivity is very high and atmospheric corrosion is
notsignificant even ordinary steel conductors shall be
permitted.(j) Earthing system with GI conductors should be
subjected to a thorough physicalexamination after a period of 10
years and modifications made if found necessary on
such examination.(k) Joints using GI conductors should be welded
as far as possible and kept separatedfrom air by a thick coating of
tar or similar non-hygroscopic materials. In case bolted
joints can not be avoided, there should be a minimum of 2 bolts
for sizes up to 25 mm x6 mm, 3 bolts for sizes up to 31 mm x 6 mm
and zig-zag bolting for large sizes.
18. Following guidelines shall be adhered to when covered
aluminium is used forearthing:-
(a) Aluminium should not be used for final underground
connections to earth electrode.(b) Aluminium shall be used below
ground only if efficiently protected or sheathedagainst contact
with soil and moisture. PVC covering shall be allowed.(c) Where a
copper conductor is to be joined to Aluminium conductor, the joints
shouldbe tinned to prevent electrolytic action.(d) Sharp bends
required in Aluminium strip should be formed by the use of a
bendingmachine.(e) Aluminium to Aluminium joints on strip conductor
shall be arc welded using eitherthe Tungsten inert gas arc or Metal
inert gas arc techniques. Oxy-acetylene gas weldingor bracing shall
also be allowed.(f) Round and rectangular conductors can be joined
with bolted clamps. Rectangularconductors can be joined or
terminated by drilling and bolting.(g) When making a bolted type
joint the surface of the Aluminium strip should becleaned
thoroughly by wire brushing and greased or an approved jointing
compoundapplied immediately to both mating surfaces. Bolts should
then be tightened and allexcess grease or compound wiped off and
discarded.(h)Aluminium conductor connections to equipment, whereas
possible, should be in thevertical plane. Surface preparation of
Aluminium and making of the joint should be aspreviously described.
The finished joint should be protected by bitumastic paint.
19. Earthing shall conform to IS 3043-1987.
20. Earthing grid should be run at a minimum depth of 50 cm
below the ground
